The present invention relates to improving alignment between flanged components during assembly, for example improving the alignment of semi-circular split casings or whole circumference stack casings such as those used in the compressor stages of a gas turbine engine.
Gas turbine engines typically employ a multi-stage axial compression system for compression of air passing through the engine. A fan at the front of the engine provides initial low pressure compression, before Intermediate and High Pressure Compressors housed within the body of the engine further increase the pressure to the levels required at the combustor.
For ease of assembly and manufacture, the generally cylindrical Intermediate Pressure Compressor (IPC) within a gas turbine engine is made up of two semi-circular split casing segments which are attached together using bolted flanges.
Typically, the two halves of the split casing structure are bolted together with the flange surfaces parallel to each other. The concentric and axial location of one half of the compressor split casing structure relative to the other half is provided only by the bolts passing through the flange bolting holes.
Similarly, advantageous assembly considerations mean that High Pressure Compressors (HPCs) are also typically formed in sections. Rather than being formed from semi-circular split casing segments, HPCs are typically assembled as a series of whole circumference stack casing segments in order to enhance the hoop strength of the casing assembly. However, the stack casing segments are still typically connected together via bolted flanges (provided around their circumference), and the rotational and concentric alignments of the stack casings relative to each other are similarly solely provided by the bolts passing through the flange bolting holes.
Both IPC and HPC casings are provided with an abradable liner which is machined, in one of the final machining processes of the casings, with the aim of achieving a tight roundness tolerance on the liner inner diameter of the casing. In order to carry out this machining process, the casings are assembled around the turning machine and the abradable liner is turned to size. The casing segments are then disassembled from each other to allow the casing to be removed from the turning machine, and are then re-assembled around the compressor drums during the engine build.
The tight liner inner diameter roundness tolerance achieved in the casing liner machining process can only be maintained if consistent location and orientation of each segment relative to the other segments in a casing assembly can be ensured for the first and subsequent assembly processes. This roundness tolerance is fundamental in achieving required compressor tip clearances, and so has a large influence on compressor efficiency. It is clearly beneficial, therefore, to ensure that the tolerances are maintained in the fully assembled engine.
Unfortunately, the required repeatability of assembly is not currently achieved on either the IPC split casings and HPC stack casings. The disassembly and subsequent re-assembly of the casing segments of either the HPC or IPC provides an opportunity for misalignment of the segments after liner machining process, due to required tolerances between the bolts and the flange bolting holes and the absence of other means of providing location of one segment of the split casing structure relative to another. The bolted flanges in the IPC do not ensure repeatability of axial or concentric alignment between the two halves of the split casing, and the bolted flanges in the HPC do not ensure repeatability of rotational or concentric alignments between the stack casings.
Curvic couplings are one known way of ensuring alignment between two components. Curvic couplings have a self-alignment tendency when the two halves of the coupling are brought together, and the square tooth profile would provide location features between the compressor segments to ensure repeatability in compressor casing segment alignment.
However, there are a number of drawbacks associated with the use of curvic couplings, a selection of which are provided below.    1. The curvic teeth are expensive and timely to machine due to the small radii associated with the square tooth profile, and a brazing process is required to attach the curvic teeth.    2. The machines required to implement such an attachment are very specialised and expensive.    3. A thick flange is required in order to allow for connection between each curvic surface. This is detrimental in terms of weight.    4. Curvic square tooth profile results in stress concentrations in the structure.    5. When the two halves of the curvic coupling are brought together there is a risk of denting the surface resulting in either a non-optimal alignment or the need for the coupling to be replaced.    6. Curvic couplings cause issues in terms of repair and overhaul. It is standard protocol to inspect every curvic coupling—this often leads to their replacement due to curvic teeth getting damaged when taken apart.
Accordingly, it is an aim of the present invention to provide an alternative solution to the problem of providing repeatable alignment between casing segments or other flanged components which overcomes or mitigates some or all of these drawbacks.